1. Field of the Invention
The present invention relates generally to an improved cordless telephone and, specifically, to a cordless telephone that automatically changes the radio-wave frequency between the handset and the base unit, in order to prevent interference with adjacent cordless telephones.
2. Description of the Prior Art
Generally, the range of movement permitted the user of a telephone is limited by the length of the telephone cord and practical considerations place a limit on such cord lengths. To overcome this problem, cordless telephones are becoming more frequently used in this country, as well as in certain foreign countries. Typically, such cordless telephone systems consist of a transmitter-receiver, in the form of a cordless handset, and a transmitter-receiver forming a so-called base unit system that is connected to the telephone line network. The handset and the base unit are coupled with each other through radio waves transmitted and received by their respective antennas.
A telephone subscriber typically uses such cordless telephone in the following manner. When calling someone, a talk button on the handset is changed from a standby position to an ON position and the call is made. Alternatively, when one wishes to receive incoming telephone calls the talk switch is placed in the standby position and when the telephone call comes in, the talk switch is changed over to the ON position. In other words, the talk switch must be in the ON state when the user intends to talk. In using a cordless telephone, the telephone subscriber can move about freely with the handset system without fear of tangling a lengthy telephone cord. The useful distance between the handset and the base unit is referred to as the service area and is typically around 300 meters, with a maximum transmission range being set by regulations of the Federal Communications Commission (FCC) The frequency of the radio waves transmitted between the handset system and the base unit system are typically in the 40 MHZ band and in the 1 MHZ band, and in most cases five duplex channels for each system are permitted by the FCC.
As described above, while great convenience is afforded the telephone subscriber is using a cordless telephone, problems arise because the handset and base unit communicate with each other through radio waves. As a result, if another cordless telephone having the same or similar frequency bands is being utilized in the immediate vicinity, such as in a neighboring house near where the subject cordless telephone is in use, radio interference will quite frequently occur between these two cordless telephone systems. If such interference with the neighboring cordless telephone does occur, it is then necessary to change the communication channel being utilized between the handset and the base unit system and this is frequently accomplished by taking the handset and base unit to a service station or repair facility where the quartz crystal oscillators in the broadcast units are replaced with ones of different frequency. Obviously this is both troublesome and disadvantageous from a time and money standpoint. Moreover, even after changing the oscillation frequencies to select new transmission channels, if the new channel is also occupied by a different cordless telephone used nearby, then the oscillator crystals must be changed once again. One proposed system for solving this problem is to change the broadcast channels of the handset and the base unit by using manually operated channel selector switches. Nevertheless, in such cases where radio interference occurs, the telephone subscriber must change not only the channel of the handset system, which is remotely arranged to the base unit, but the user must go back to the base unit and change the channel there to agree with that to which the handset was changed. This obviously reduces the effectiveness of a cordless telephone.